Maybe you could explain it's a backend for an editor.
Or maybe "unstyled"?
Unless I misunderstood the example, all you have to do is instantiate it to make it work with a default look.
If "using the API" is headless, then everything is headless because they all have an API, including things like Qt, WxWidgets, etc. This is obviously not the case.
The package comes without CSS and markup as explained on the frontpage.
> Headless > It’s headless and comes without any CSS. You are in full control over markup, styling and behaviour
I saw the title of this story and assumed that tiptap was going to be a component that somehow backed up various WYSIWIG editor-clients. That would be headless. (It's even useful -- the transaction system + collaboration server is theoretically a thing that could back up other clients, after all, though in practice it's going to force enough design decisions onto the clients that it's not really worth it.)
Tiptap does come with a graphical user interface, insofar as it looks like the most minimal possible use of it (from your "Minimal setup" example) will create a contenteditable surface to write in. Said surface is the "head".
I'd consider "flexible", "customizable", or "unopinionated". "Unstyled" was suggested, but although it's true I think it's misleading -- the need to construct your own toolbar goes beyond what that implies.